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- Andersson, C., et al.
(författare)
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User-friendliness and transparency in PSA modelling
- 2010
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Ingår i: Proc. of PSAM 10 — International Probabilistic Safety Assessment & Management Conference, 7–11 June 2010, Seattle, Washington, USA, IAPSAM — International Association of Probabilistic Safety Assessment and Management.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Most of the probabilistic safety assessments (PSA) for Nuclear Power Plants were originally created to make conservative estimates of the core damage frequencies for internal events, which might occur during power operation. The PSA models have then been expanded to replace conservative estimates with more realistic assumptions and to include other types of initiating events, modes of operation and end states. The development has resulted in very large and detailed models, which are hard to understand completely, even for an experienced PSA engineer. Today, the trend to increase the level of detail and the scope of the PSA models continues as a consequence of regulatory requirements. Hence the Nordic PSA Group (NPSAG) has initiated a project with the aim of identifying methods to reduce the complexity of the PSA models. This paper presents and discusses the results of the first part of the project in which areas of importance for the user-friendliness and transparency of a PSA are identified.
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| 4. |
- Bengtsson, Lisa, et al.
(författare)
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Probabilistic Safety Goals for Nuclear Power Plants; Phases 2-4 : Final Report
- 2011
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Safety goals are defined in different ways in different countries and also used differently. Many countries are presently developing them in connection to the transfer to risk-informed regulation of both operating nuclear power plants (NPP) and new designs. However, it is far from self-evident how probabilistic safety criteria should be defined and used. On one hand, experience indicates that safety goals are valuable tools for the interpretation of results from a probabilistic safety assessment (PSA), and they tend to enhance the realism of a risk assessment. On the other hand, strict use of probabilistic criteria is usually avoided. A major problem is the large number of different uncertainties in a PSA model, which makes it difficult to demonstrate the compliance with a probabilistic criterion. Further, it has been seen that PSA results can change a lot over time due to scope extensions, revised operating experience data, method development, changes in system requirements, or increases of level of detail, mostly leading to an increase of the frequency of the calculated risk. This can cause a problem of consistency in the judgments.This report presents the results from the second, third and fourth phases of the project (2007–2009), which have dealt with providing guidance related to the resolution of some specific problems, such as the problem of consistency in judgement, comparability of safety goals used in different industries, the relationship between criteria on different levels, and relations between criteria for level 2 and 3 PSA. In parallel, additional context information has been provided. This was achieved by extending the international overview by contributing to and benefiting from a survey on PSA safety criteria which was initiated in 2006 within the OECD/NEA Working Group Risk.The results from the project can be used as a platform for discussions at the utilities on how to define and use quantitative safety goals. The results can also be used by safety authorities as a reference for risk-informed regulation. The outcome can have an impact on the requirements on PSA, e.g., regarding quality, scope, level of detail, and documen¬tation. Finally, the results can be expected to support on-going activities concerning risk-informed applications.The project provides a comprehensive state-of-the-art description and has contributed to clarifying the history of safety goals both nationally and internationally, the concepts involved in defining and applying probabilistic safety criteria, and the international status and trends in general. It has identified critical issues and the main problem areas. Finally, the project provides useful recommendations and guidance on the definition and application of criteria.Furthermore, the project makes it possible to define criteria stringently, improving the possibilities of argumentation on safety. Generally, this supports efficient use of criteria, yielding more useful PSA results. In this connection, the introduction of ALARP type criteria is judged to provide a very useful way of balancing stringency with the necessary flexibility. There is a possibility of making more active use of lower level criteria. This makes the connection to defence in depth more evident, and opens the perspective of increased control of defence in depth by use of probabilistic methods, including the use as design tools. There is an opportunity for comparison of risk of different NPPs, as well as of comparison of NPP risk with other risks in society. This is judged to provide an opportunity for improved communication on risks with non-PSA experts and with the public in general. However, a necessary condition for meaningful comparisons is to agree on the scope of PSA and methods applied.Obviously, there will also be challenges in the future definition and application of probabilistic safety criteria. These include very general aspects, such as the interpretation of the probability, quality aspects of PSA, and the definition of meaningful and consistent risk criteria for different usages. The need and usefulness of subsidiary criteria has been stressed in the project, but there is obviously also a challenge in defining a relevant set of criteria on different levels. Defining criteria for L(E)RF is complex, especially if release criteria are defined as subsidiary for societal and individual risk. Finally, it will be a challenge to develop coherent application procedures relative to the criteria defined.
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| 6. |
- Bladh, Kent, et al.
(författare)
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Enhanced Bayesian THERP : Experience from HRA method evaluation
- 2010
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Ingår i: Proceedings of European Safety and Reliability, ESREL 2010. - London : Taylor & Francis Group.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Enhanced Bayesian THERP (Technique for Human Reliability Analysis) method uses as its basis the time-reliability curve introduced in the Swain’s human reliability analysis (HRA) handbook. It differs from the Swain's Handbook via a transparent adjustment of the time-dependent human error probabilities by use of five performance shaping factors (PSFs): (1) support from procedures, (2) support from training, (3) feedback from process, (4) need for co-ordination and communication, (5) mental load, decision burden. In order to better know the characteristics of the Enhanced Bayesian THERP from a more international perspective, the method has been subject to evaluation within the framework of the international “HRA Methods Empirical Study Using Simulator Data”. Without knowledge of the crews’ performances, several HRA analysis teams from different countries, using different methods, performed predictive analyses of four scenarios. This paper gives an overview of the method with major findings from the benchmarking. The empirical comparison gives confidence that the time reliability curve is a feasible and cost effective method to estimate human error probabilities when the time window is well defined and relatively short. The comparison of empirical observations with predictions was found as an useful exercise to identify areas of improvements in the HRA method.
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| 7. |
- Hessel, Phillippe, et al.
(författare)
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Status and Experience with the Technical Basis and Use of Probabilistic Risk Criteria for Nuclear Power Plants
- 2010
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Ingår i: Proc. of PSAM 10.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Probabilistic safety criteria, including safety goals, have been progressively introduced byregulatory bodies and utilities. They range from high level qualitative statements to technical criteria.They have been published in different ways, from legal documents to internal guides. They can beapplied as legal limits down to “orientation values”. The OECD/NEA Working Group on Risk(WGRISK) prepared a questionnaire on the probabilistic risk criteria for nuclear power plants.Answers were received from 13 nuclear safety organizations and 6 utilities. The reported probabilisticr i s k c r i t e r i a c a n b e g r o u p e d i n t o 4 c a t e g o r i e s , i n r e l a t i o n w i t h t h e t o o l s t o b e u s e d f o r a s s e s s i n gcompliance: core damage frequency, releases frequency, frequency of doses and criteria oncontainment failure. Introduction of probabilistic safety criteria is generally considered to result insafety improvements. Opinion is widespread on the benefits of using probabilistic safety criteria forcommunication with the public, ranging from bad to good experiences. The responses to thequestionnaire suggested that more work should be considered in the definition of releases frequencies:some regulators include a time range (generally 24 hours) in the criterion while others do not limit thet i m e t o b e c o n s i d e r e d . I t i s s u g g e s t e d t h a t , i n t h e f i r s t c a s e , t h e e x i s t i n g P S A s s h o u l d b e r e v i s i t e d t oassess if long development accident sequences were considered
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| 8. |
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| 9. |
- Holmberg, Jan-Erik, 1964-, et al.
(författare)
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Development of methods for risk follow-up and handling of CCF events in PSA applications
- 2010
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Ingår i: Proc. of PSAM 10 — International Probabilistic Safety Assessment & Management Conference, 7–11 June 2010, Seattle, Washington, USA, IAPSAM — International Association of Probabilistic Safety Assessment and Management. - : International Association for Probabilistic Safety Assessment and Management. - 9781450715560 ; , s. 53-
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Konferensbidrag (refereegranskat)abstract
- Risk follow-up aims at analysis of operational events from their risk point of view usingprobabilistic safety assessment (PSA) as the basis. Risk follow-up provides additional insight tooperational experience feedback compared to deterministic event analysis. Even though thisapplication of PSA is internationally widely spread and tried out for more than a decade at manynuclear power plants, there are several problematic issues in the performance of a retrospective riskanalysis as well as in the interpretation of the results.An R&D project sponsored by the Nordic PSA group (NPSAG) has focused on selected issues in thistopic. The main development needs were seen in the handling of CCF and the reference levels forresult presentation. CCF events can be difficult to assess due to possibilities to interpret the eventdifferently. Therefore a sensitivity study with varying assumptions is recommended as a generalapproach. Reference levels for indicators are proposed based on the survey of criteria usedinternationally. The paper summarizes the results.
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| 10. |
- Holmberg, Jan-Erik, 1964-, et al.
(författare)
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Enhanced Bayesian THERP : Lessons learnt from HRA benchmarking
- 2010
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Ingår i: Proc. of PSAM 10 — International Probabilistic Safety Assessment & Management Conference, 7–11 June 2010, Seattle, Washington, USA, IAPSAM — International Association of Probabilistic Safety Assessment and Management. - : International Association for Probabilistic Safety Assessment and Managemen. ; , s. 52-
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Konferensbidrag (refereegranskat)abstract
- The Enhanced Bayesian THERP (Technique for Human Reliability Analysis) method usesas its basis the time-reliability curve introduced in the Swain’s human reliability analysis (HRA)handbook. It differs from the Swain's Handbook via a transparent adjustment of the time-dependenthuman error probabilities by use of five performance shaping factors (PSFs): (1) support fromprocedures, (2) support from training, (3) feedback from process, (4) need for co-ordination andcommunication, (5) mental load, decision burden. In order to better know the characteristics of theEnhanced Bayesian THERP from a more international perspective, the method has been subject toevaluation within the framework of the international “HRA Methods Empirical Study Using SimulatorData”. Without knowledge of the crews’ performances, several HRA analysis teams from differentcountries, using different methods, performed predictive analyses of four scenarios. This paper givesan overview of the method with major findings from the benchmarking. The empirical comparisongives confidence that the time reliability curve is a feasible and cost effective method to estimatehuman error probabilities when the time window is well defined and relatively short. The comparisonof empirical observations with predictions was found as a useful exercise to identify areas ofimprovements in the HRA method.
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